A sensor that measures physical quantities such as temperature and strain using an optical fiber have some advantages such as a long operating life, a lightweight, a thin diameter and a flexibility, and so it can be used in narrow spaces. In addition, this sensor has a characteristic of a strong resistance to electromagnetic noise due to insulation property of the optical fiber. For that reason, this sensor is expected to be used in structural health monitoring of large constructions such as bridges and buildings, and aerospace equipment such as passenger airplanes and manmade satellites.
Performance requirements of the sensor for applying the structural health monitoring in these structures include high strain resolution, high spatial resolution, in-sensor strain distribution measurement capability, having a multipoint (multiplexed) sensor (a wide detection range), and a capability of real-time measurement, and the like.
Although various optical fiber sensor systems have been previously proposed, an optical fiber sensor using an FBG sensor and the OFDR type analysis method is regarded as the most promising optical fiber sensor that sufficiently satisfies the above-mentioned performance requirements.
The optical fiber sensor system that uses the FBG sensor and the OFDR type analysis method determines the position of the FBG sensor using cyclical change in the interference light intensity between the Bragg reflected light from the FBG sensor and reflected light from the referential reflecting end (reflecting end for reference). In addition, this optical fiber sensor system measures strain and temperature of the detection portion from the change amount of the wavelength of the Bragg reflected light.
Hitherto disclosed examples of this optical fiber sensor system include one that is capable of measuring strain distribution in a sensor with high strain resolution (for example, refer to Non-Patent Literature 1 and Patent Literature 3), one that has a high spatial resolution of 1 mm or less (for example, refer to Non-Patent Literature 2), one in which eight hundred FBG sensors are multiplexed on an eight-meter optical fiber, and one can measure strain at more than three thousand points with a total of four optical fibers simultaneously (for example, refer to Non-Patent Literature 3), and one can real time measurements (for example, refer to Patent Literature 1). Here, in-sensor strain distribution measurement that is disclosed in Non-Patent Literature 1 and Patent Literature 3 means being able to measure non-uniform strain that occurs along the long direction of the FBG sensor.
A general problem of optical fiber sensor systems includes that, when there is change in a plurality of items of physical quantity such as temperature and strain, it is not possible to independently identify and measure amount of these changes. For that reason, for example, in the case of using an optical fiber sensor system as a strain sensor, a separate temperature-compensating sensor must be used so that temperature change of a detection portion is not treated as the change in strain.
To solve this problem, a method using FBG sensors that consist of a PM fibers has been proposed (for example, refer to Patent Literature 2). In this method, PANDA type PM fiber is used for FBG sensor, and temperature and strain can be measured by measuring the amount of change in the wavelength of Bragg reflected lights from two orthogonal polarization axes at the FBG sensor consists of this PANDA fiber.
That is, this method provides a strain sensor that does not require a temperature-compensating sensor.
Conceivably, if the technologies mentioned above are combined in an optical fiber sensor system using FBG sensors consist of PM fiber and OFDR type analysis method; it will be possible to achieve high strain resolution, high spatial resolution, multi-point measuring, real-time measuring, and simultaneous measurement of temperature and strain.
[Patent Literature 1] Japanese Patent No. 3740500
[Patent Literature 2] Japanese Patent No. 3819119
[Patent Literature 3] Japanese Patent No. 4102291
[Non-Patent Literature 1] H. Igawa, H. Murayama, T. Kasai, I. Yamaguchi, K. Kageyama and K. Ohta, “Measurement of strain distributions with long gauge FBG sensor using optical frequency domain reflectometry” Proceedings OFS-17, pp. 547-550 (2005)
[Non-Patent Literature 2] H. Murayama, H. Igawa, K. Kageyama, K. Ohta, I. Ohsawa, K. Uzawa, M. Kanai, T. Kasai and I. Yamaguchi, “Distributed Strain Measurement with High Spatial Resolution Using Fiber Bragg Gratings and Optical Frequency Domain Reflectometry” Proceedings OFS-18, ThE40 (2006)
[Non-Patent Literature 3] B. Childers, M. E. Froggatt, S. G Allison, T. C. Moore, D. A. Hare, C. F. Batten and D. C. Jegley, “Use of 3000 Bragg grating strain sensors distributed on four eight-meter optical fibers during static load test of a composite structure” Proceedings SPIE's 8th International Symposium on Smart Structure and Materials, Vol. 4332, pp. 133-142 (2001)